The Arctic Ocean covers only about 3% of the global ocean areas, but it is responsible for up to 14% of the global oceanic carbon uptake. This makes it an important sink for atmospheric carbon dioxide. As sea ice is dramatically shrinking in the Arctic Ocean, the ecological consequences of its decline, including changes in the carbon cycle, are still largely unknown. In a study recently published in the renowned scientific journal Science Advances, senior researcher Letizia Tedesco from the Marine Research Centre of the Finnish Environment Institute, together with Associate Professor Marcello Vichi from the Department of Oceanography at the University of Cape Town (South Africa), and Enrico Scoccimarro, senior researcher at the CMCC Foundation, CSP – Climate Simulation and Prediction Division in Bologna (Italy), investigated how the anticipated snow and sea ice changes would impact sympagic production in the Arctic Ocean. They combined a biogeochemical model for sea ice algae with an ensemble of climate model results from a “business as usual” climate change scenario (RCP8.5), which is the one assuming no mitigation measures.

“The Arctic sea-ice decline is among the most emblematic manifestations of climate change and is occurring before we understand its ecological consequences”, researcher Enrico Scoccimarro explains. “In this joint modelling study of the University of Cape Town – Department of Oceanography, the Marine Research Centre of the Finnish Environment Institute and the CMCC Foundation we underlined that in a warmer climate, changes in primary productivity of sympagic algae are not linear with latitude and the main causes responsible for the projected changes are different at different latitudes”.

Phytoplankton are tiny algae floating and drifting in the oceans. Like plants, phytoplankton are primary producers, i.e., transforming carbon dioxide into oxygen, and are at the very base of the marine food web. They are the main source of food for zooplankton, which are the main diet for larger zooplankton, sea birds, fishes, up to seals, whales, and polar bears in the Arctic Ocean. Though they are visible only by microscope inspection, they are vital to life in the oceans and more generally to life on Earth, since they are responsible for fixing about 50% of the oxygen on this planet, otherwise the global concentration of carbon dioxide would be about 50% higher.

Sea ice algae are types of phytoplankton found in seasonal and perennial sea ice. Sea ice is made of a solid matrix of pure ice and a liquid fraction of salty brines, where sea ice algae live. Like all phytoplankton, sea ice algae are primary producers, but they do not float in the water like other phytoplankton. They live in the limited space of the brine channels and pockets within sea ice. Sea ice algae play a primary role in the marine food web of polar oceans. The sea ice algae are highly adapted to the extreme environment they live within. They can grow earlier in the arctic spring than other phytoplankton, which means they extend the productive season in polar oceans. During the time when there is not enough light for the phytoplankton to grow in the ocean, the sea ice algae provide the only food source available to the rest of the food chain in the Arctic Ocean.

By using model outputs at the highest available temporal resolution, the authors found almost linear physical changes along latitudes, such as snow and ice thinning and ice season shortening, when comparing the time period 1961-2005 with 2061-2100. They also found a worrying decrease of seasonal sea ice below 70˚N and a striking increase of seasonal sea ice extent at the expenses of multi-year ice above 70˚N. They then investigated whether an advance in melt onsets would be associated to a similar advance in algal bloom onsets, and whether the changes in seasonal sea ice extent would follow the loss and gain of the seasonal sea ice habitat. As it often happens in nature, a much more complex response of sympagic algae was found by the authors. “Below 66˚N”, Enrico Scoccimarro says, “we found that the thinning of the snow cover was so pronounced that the sympagic algae were growing very early, and there was a clear mismatch between melt onset and bloom onset. At 66-74˚N latitudes, we found that earlier ice melting was setting an upper limit to the accumulation of biomass, causing little changes despite the much earlier bloom onset. Finally, above 74˚N, the model indicated that the algal bloom period would shift earlier into a more light-favourable time of the year, i.e. from fall to summer, causing the largest of all relative increases in primary production, up to more than 2000%. Overall, model projections suggested a relative increase of almost 50% in sympagic primary production on a pan-Arctic scale along this century, but with contrasting spatial patterns”.

These diverse latitudinal responses indicate that the impact of declining sea-ice on Arctic sympagic production is anticipated to be large and also complex, as well as the expected trophic (i.e., changes in abundances) and phenological (i.e changes in timing) cascades (i.e. impacting adjacent trophic levels) on the rest of the food web. The phenological synchrony across trophic levels is expected to be driven by the pace of earlier melt onsets and consequent shift in algal bloom timing. Thus, we might expect the strongest phenological cascades at the lowest latitudes where algal blooms were projected to occur significantly earlier, while the largest trophic cascades at the highest latitudes where the largest relative increase in biomass was projected. Since the Arctic marine food web is short, poorly diverse and seasonally driven by limited pulses of energy, the expected trophic cascades and phenological shifts have the potential to heavily impact also on higher trophic levels, from fish stock abundances to food availability for whales, seals and polar bears, eventually putting endemic (i.e. sea-ice dependent) top predators under risk of trophic and phenological mismatches fundamental to their survival.

The CMCC Foundation contributes to the study and provides also the physical forcings used to drive the sea-ice biogeochemical model. The CMCC-CM and CMCC-CMS models cited in the study contributed to the CMIP5 project, providing the basis for the IPCC Fifth Assessment Report (AR5) recommendations; the CMCC Foundation is now involved in the realization of the scenario simulations for CMIP6 to be used for the upcoming IPCC Sixth Assessment Report (AR6).